Machinery's Handbook, 31st Edition
2506 VISCOSITY at 100 ° F (40 ° C) and 212 ° F (100 ° C), then its results are compared with two or more series of oils. VI values once ranged between 0 and 100, but recent developments in lubricant technology and additives have allowed this index to raise its upper limit and include a Very High Viscosity Index (VHVI) group. Lubricants are generally classified in four basic VI groups depicted in Table 3. Table 3. Viscosity Index Rating VI rating Viscosity Index Group < 35 Low (LVI) 35–80 Medium (MVI) 80–110 High (HVI) > 110 Very High (VHVI) Composition of Oil.— Oil is composed of either a mineral (hydrocarbon based) or syn thetic oil base stock to which is added a variety of organic and inorganic compounds that are dissolved or suspended as solids in the formulated oil. Depending on the end use condition the oil formulation is designed for, the additive package can make up from 1% up to 30% of the formulated oil volume. Mineral-Based Oil.— Mineral oils are refined from crude oil stocks. Depending on where the crude stock is found in the world, the oil can be paraffinic or napthenic based. Paraffinic based stocks are generally found in the midcontinental United States, En- gland’s North Sea, and the Middle East. They contain a 60/30/10 mix of paraffin/nap - thene/wax resulting in a very high VI rating up to 105. Because wax is present, they are known to have a wax pour point in which the oil’s flow is severely constricted or stopped by wax crystallization at lower temperatures. This type of base oil stock is preferred when blending high quality crankcase oils, hydraulic fluids. turbine oils, gear oils, and bearing oils. Napthenic based oil stocks are generally found in South America and the coastal regions of the United States. They contain a 25/75/trace mix of paraffin/napthene/wax, resulting in a less stable VI rating up to 70. Because only a trace of wax is present they are known as viscosity pour point oils in which oil flow is restricted by increases in the lubricant’s viscosity at low temperatures. Napthenic oils have lower pour points, higher flash points, and better additive solvency than paraffinic oils. This type of base stock is preferred when blending locomotive oils, refrigerant, and compressor oils. Oil Additives.— When contact is likely between two bearing surfaces the lubricant should be designed to mitigate the friction through the addition of engineered additives to the base oil. Every manufactured lubricant on the market has its own unique formulation. In effect, it is an engineered liquid, custom built to perform a specific a job in a specific environment. All additives are sacrificial and therefore careful attention to additive pack - age levels through the use of oil analysis will tell the user exactly when to change the oil to prevent damage to the bearing or contact parts. Typically oil additives as shown in Table 4 are used to enhance the existing base oil, add additional properties to the oil, and suppress any undesirable properties the base oil may have. Table 4. Oil Additives Enhancement Additives New Property Additives Suppressant Additives Anti-oxidant EP Pour point depressant viscosity improver Corrosion inhibitor Anti-wear Demulsifier Detergent Anti-foam Dispersant The additive package formulation will depend on the end use. Table 5 references what oil type generally carries what additive package in its formulation.
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